I have a m101a3 - second picture excepted converted for humvee. Pros: sturdy, surge brakes, heavy duty stand; will hold a lot of wood with sideboards, neat tailgate, easy 12 volt conversion and easily converted to tilt for unloading (not actually a dump but with some money and a lift mechanism it could be a dump, Tilt ability is accomplished by giving brake lines room to swing and replacing two bolts with pins; to tilt remove the pins, set the trailer brakes and slowly backup.).
Cons: sideboards are fiberglass and not really up to firewood chucking, loading height is high, lights require conversion though simple, pintle hitch is huge, axle swap required to reconvert from humvee setup, and bed is short.
Cost varies. Straight from government can be cheap. I brought 4 really nice ones for a total of less than $1,000. Traded one for the 600+ mile round trip to military base. Sold two within days at $600 each and kept one. Listing retail around here is anywhere from $900 to $1200. They are getting harder to find as government switched to the style shown in the first picture. These run from $1500 to $2500. I am told the beds are aluminum.
The larger ones (m105) for the 2.5 ton 6x6 trucks are too big in my opinion. Same cons plus air over hydraulic brakes and heavy wheels and tires. The last version, however, had surge brakes and lighter wheels and tires. I had one. Towed it home with my 1993 Toyota Landcruiser. Would have overwhelmed the Toyota if loaded. Worked nice on the deuce for which it was designed. I would not recommend any version of the m105 for firewood.
Ron